Systemic chemotherapy is a vital component of breast cancer treatment, but early-onset toxicities like neutropenia hinder its administration, especially in the elderly. Primary prophylactic (PP) use of granulocyte colony-stimulating factors (G-CSFs) prevents neutropenia and improves successful administration of life-saving chemotherapy. PPG-CSF use has considerably increased since 2003, after the introduction of scientific-evidence demonstrating the benefits of PPG-CSF supported dose-dense chemotherapy in breast cancer patients. Consequently, PPG-CSF has become a vital component of breast cancer care, especially in the elderly. However, G-CSFs are expensive drugs. Every year approximately $3.4 billion are spent nationally on G-CSFs. The two most commonly used G-CSFs in the US are the short-acting filgrastim and the long-acting pegfilgrastim. Pegfilgrastim, given as a single dose per chemotherapy cycle, is easier to administer as compared to filgrastim, which is administered daily for 5 to 7 days per chemotherapy cycle in real-life clinical settings. However, pegfilgrastim is also 11 times more expensive than filgrastim. In spite of numerous studies exploring the comparative effectiveness of PP pegfilgrastim and PP filgrastim over the past decade certain clinical questions remain unanswered. First, there is considerable ambiguity over the comparative effectiveness of pegfilgrastim and filgrastim, and debate over the exact filgrastim duration-equivalent of a single pegfilgrastim dose for preventing neutropenia. Second, the actual costs and long-term survival benefits associated with the two drugs in real-life clinical-settings are unknown. This dearth of information is more pronounced in the elderly due to their exclusion from most clinical trials. Currently, G-CSF expenditure is the fifth largest individual Medicare Part B drug expenditure, accounting for $494 million/year. Given the Medicare cost burden of the G-CSF drugs and their vital role in breast cancer treatment among the elderly, it is important to understand the clinical outcomes, costs and long-term benefits associated with these drugs in the elderly breast cancer population. This study aims to estimate the comparative effectiveness of PP pegfilgrastim versus PP filgrastim in preventing neutropenia, reducing Medicare costs, and improving long-term survival benefits in elderly breast cancer patients receiving first-course chemotherapy. In addition, the study aims to estimate the filgrastim duration-equivalent of a single pegfilgrastim dose for preventing neutropenia. The study proposes to address these aims by analyzing the nationally representative SEER-Medicare data for women 66 years and above, newly diagnosed with stage I to III breast cancer between the years 2003 to 2007, and receiving first-course chemotherapy.